Flame suppressants are classified as either active (chemical) or passive (physical) suppressants. Active suppression agents react chemically with and destroy free radicals in the flame. Free radicals are very short-lived species that catalyze flame reactions. Their removal by the action of potassium salts, particularly halides, may be used to extinguish flames and even to reduce the secondary muzzle flash of guns.
One form of active suppressant is a class of materials called Halon™, which are composed of brominated or chlorinated fluorocarbon compounds, e.g., bromochlorodifluoromethane (CF2BrCl) and trifluorobromomethane (CF3Br). Halon™ materials have been used effectively as fire suppression agents for years, typically to protect electrical equipment since there is very little residue to clean up. Halon™ fire suppression agents typically interrupt the chemical reaction that takes place when fuels burn and depend on a combination of chemical effectiveness, e.g., quenching of free radicals, and some physical effectiveness, e.g., cooling the combustion flame and dilution of the combustion ingredients. Certain halogen-containing fire suppression agents, however, such as CF3Br, contribute to the destruction of stratospheric ozone. Although Halon™ materials are essentially nontoxic, passage through a flame or over hot surfaces produces some very toxic fluorine compounds.
To reduce the environmental effects associated with Halons™, most commercially available fire suppression agents designed today are passive, i.e., physically acting, agents. A passive suppressant does not react chemically with the flame. These fire suppression agents either blanket the burning material to deprive it of oxygen, or they dilute the oxygen in the environment to below the point that can sustain the flame, or they cool the burning surface below its ignition temperature.
Examples of physically-acting fire suppression agents include sodium bicarbonate and sand as well as inert gases, e.g., carbon dioxide (CO2), water vapor (H2O), and nitrogen (N2). When applied to a fire, inert gases physically displace oxygen from the combustion region while simultaneously serving as a heat sink to reduce the temperature of the flame. The combination of the two physical actions results in suppression of the fire. Gaseous passive agents cannot be used as total flooding agents in occupied spaces because they must reduce the oxygen content below the amount that will sustain life. This is especially true for carbon dioxide because it also interferes with human respiration at high concentrations.
Unfortunately, physically-acting fire suppression agents tend to be less efficient than chemically-acting fire suppression agents. Accordingly, a larger quantity of a physically-acting fire suppressant is required in order to suppress a fire and, consequently, equipment and storage must be large to accommodate the large quantity. Such large equipment is a disadvantage in limited spaces. Applications in which space and weight are limited include military or civilian aircraft or ground vehicle engine bays, automobiles, spacecraft, or military or civilian aircraft drybays. Another disadvantage of dry physical suppressants is their particle size, which requires physical blowing or shoveling to emplace them. The large size of the particles also prevents penetration of the agent to combustion areas which are concealed or relatively inaccessible.
As a result, relatively small areas are typically equipped with handheld fire extinguishers that require a person to operate. Because aircraft cargo bays and cargo containers on ships and trains are generally left unmonitored, a fire in these areas can become serious before anyone becomes aware that the fire even exists. The spread of fire from these relatively small areas can result in the loss of the entire vehicle. Thus, current fire suppression methods in such areas depend on human intervention, providing that such intervention occurs promptly enough to prevent the fire from spreading and causing large scale damage.
An advantageous alternative to the above suppressant agent systems is the use of a pyrotechnically-generated aerosol flame free radical suppressant. This generation method may provide such fine particles that their free-fall velocity is less than the velocity of air currents in an enclosed space. As such, the particles stay suspended in the exhaust of the pyrochemical generator, and seek out even concealed fires such as those that might be found inside aircraft cargo subcontainers, such as the LD-3 container used on commercial aircraft. The smoke-like suspension characteristics of the aerosol provide long “hang times,” referring to the length of time a single generator function can continue to suppress recurrent flame. Another advantage of such pyrochemically generated aerosol is that their ozone-depleting potential may approach zero, that their inhalation toxicity may be much lower than that of inert gas, and that no toxic irritant gases may be generated on passage through flame or with hot surfaces.
The use of currently known pyrotechnic flame suppressant aerosol generating compositions as can be problematic. For example, such aerosol generating compositions have some thermal stability problems and are significantly sensitive to accidental ignition by mechanical impact or friction. This sensitivity poses a safety concern in their manufacture, storage and use.
Prior art aerosol generating flame suppressants typically produce unduly hot and destructive gases. Such gases may include permanent gases and suppressant vapor prior to its condensation to an aerosol, the form in which the flame suppressant is delivered. If these gases are not cooled, structures, machinery, cargo and living beings may be damaged. In fires in an enclosed space, hot gases rapidly rise and can carry an aerosol flame suppressant up above a low-lying fire, where it cannot extinguish the fire.
The use of solid coolants, however, condenses and traps at least a portion of the aerosol generating flame suppressant, rendering it ineffective in putting out the flames. As a result, it is necessary to use a larger amount of aerosol generating flame suppressant, which detrimentally produces additional heat and destructive gas. Moreover, solid coolants are heavy and voluminous, often being two or six times the weight and volume of the aerosol generating flame suppressant. In addition, the coolants often produce toxic gases, such as carbon monoxide, to the peril of nearby persons.
As such, there is a need in the art for clean, effective, non-toxic, non-ozone depleting, and inexpensive fire extinguishing agents.